The present invention relates to valve operating apparatus for an internal combustion engine. More particularly, the invention relates to apparatus effective to vary the operational characteristics of intake or exhaust valves in an internal combustion engine during various operating ranges of the engine.
The combustion chambers of a four-cycle engine have intake and exhaust valves for supplying an air-fuel mixture into, and discharging a burned gas from, the combustion chambers according to prescribed cycles. These intake and exhaust valves are normally urged in a closing direction by valve springs disposed around the respective valve stems. The intake and exhaust valves are adapted to be forcibly opened against the bias of the valve springs by cams integrally formed on a camshaft which is driven by the crankshaft of the engine through a belt and pulleys, or the like.
There have been proposed various devices wherein a plurality of intake valves or exhaust valves are disposed in each cylinder of an engine, and wherein, during low-speed operation of the engine, one of the intake or exhaust valves is operated, while, during high-speed operation, all of the valves are operated, and, at the same time, the operation timing of the valves is varied dependent on the engine rotational speed. With this arrangement, the efficiency with which an air-fuel mixture is charged into a combustion chamber can be increased over a wide range of engine operation. One such device is described in Japanese Laid-Open Patent Publication No. 61-19911 that is assigned to the assignee of the present application. This prior art publication discloses, as a device for changing the valve operation timing, a device for disabling the valve operation of an internal combustion engine, including a camshaft rotatable in synchronism with the rotation of an engine and having an integral low-speed cam aligned with one of the intake or exhaust valves and having a cam profile corresponding to low-speed operation of the engine and an integral high-speed cam having a cam profile corresponding to high-speed operation of the engine, a rocker shaft, a first rocker arm angularly movably supported on the rocker shaft and held in sliding contact with the low-speed cam and engageable with said one intake or exhaust valve, a second rocker arm angularly movably supported on the rocker shaft and engageable with the other intake or exhaust valve, a third rocker arm held in sliding contact with the high-speed cam, the first, second, and third rocker arms being relatively angularly displaceable in mutual sliding contact, and having coupling means for selectively interconnecting the first, second, and third rocker arms and allowing these rocker arms to be relatively angularly displaced. The coupling device includes pistons slidably fitted in guide holes disposed in the rocker arms in mutual communication, the pistons being hydraulically operable to interconnect the rocker arms, as disclosed in the specification of the above publication. In this known structure, the third rocker arm, which is held in sliding contact with the high-speed cam, is angularly moved by that cam at all times. The third rocker arm transmits the lifting movement of the cam to the valves, however, only when the rocker arms are interconnected. When the rocker arms are disconnected, the third rocker arm moves only idly.
Characteristically in such devices, while the first and second rocker arms held in sliding contact with the low-speed cam are always kept in sliding contact with that cam by valve springs, the third rocker arm is maintained in sliding contact with the high-speed cam by a lifter device when the rocker arms are disconnected. The biasing force imposed by the lifter device is, therefore, of a critical nature. More specifically, if this biasing force is too small, it cannot limit the inertial motion of the rocker arms during high-speed operation. If the biasing force is too large, on the other hand, increased friction due to the pushing force against the third rocker arm will result. In particular, it has been found that, since the coupling device is operated at the base-circle portions of the cams, undue biasing forces applied at the base-circle portions of the cams will accelerate wear on the high-speed cam or the cam slipper of the high-speed rocker arm at a rate higher than the rate of wear on the low-speed cam.
In view of the aforesaid drawbacks of the prior art, it is a primary object of the present invention to provide a device for changing the valve operation timing of an internal combustion engine, the device being improved so as to be capable of appropriately limiting biasing forces applied to an idly moving rocker arm for pushing the same against a cam surface.